Switch tracing to new flexbox.

tracing/tracing/ui/tracks/process_track_base.css

 /* Copyright (c) 2013 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 .process-track-header {
-  -webkit-flex: 0 0 auto;
+  flex: 0 0 auto;
   background-image: -webkit-gradient(linear,
                                      0 0, 100% 0,
                                      from(#E5E5E5),
                                      to(#D1D1D1));
   border-bottom: 1px solid #8e8e8e;
   border-top: 1px solid white;
   font-size: 75%;
 }
 
 .process-track-name:before {
   content: '\25B8'; /* Right triangle */
   padding: 0 5px;
 }
 
 .process-track-base.expanded .process-track-name:before {
   content: '\25BE'; /* Down triangle */
 }